5.1 Classifying Rocks

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Vocabulary

Grains: Particles of minerals or other rocks
Igneous Rock: It forms from the cooling of molten rock - either magma below the surface or lava at the surface
Sedimentary Rock: It forms when particles of other rocks or the remains of plants and animals are pressed and cemented together
Metamorphic Rock: It is formed when an existing rock is changed by heat, pressure, or chemical reactions

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Outline

How Geologists Classify Rocks

• Earth's crust is made of rocks and is used to make mountains, hills, valleys, and beaches
• Geologist collect samples of rock and study them to classify the rock
  • In order to obtain a sample, geologists chisel a piece of rock out of the place where it was found
  • After the sample is obtained, geologist crack open the rock to see what is inside
    • Geologist look in the inside of the rock because the outside of a rock can be changed by weathering
  • While studying the sample, geologists note the rock's color and texture to find out its mineral composition
  • Using these characteristics, geologists can designate a rock based on its origin, or where the rock has and where it formed

Texture

• Texture by itself doesn't classify a rock, but it does give some vital information to classify it
• A rock's texture is how the rock looks and feels
  • Some rocks have a glassy or smooth texture while others have a rough or chalky texture
• Rocks are made with tiny particles of other rocks and minerals called grains
• The grains of the rock are what give the rock its texture
• Geologist use a number of terms to describe the size, shape and pattern of the rock's grains
  • Grain Size
    • The grains in some rocks are so big that are very easy to see and are called coarse grained
    • However, there are rocks with grains so small that they are invisible
    • These rocks are fine grained
  • Grain Shape
    • The grains in a rock vary widely from the grains in other rocks
      • The grains could look like tiny particles of sand to small seeds and shooting stars
    • In other cases, the shape of the crystal could shape the grains
    • In other rocks, the grains result from fragments of other types of rock
  • Grain Pattern
    • The patterns that rocks often form, form from the grains in a rock
      • Some rocks look like stacks of pancakes or also like rows of multicolored beads while others form wavy, swirling patterns
      • In some rocks the grains occur randomly throughout the rock
  • No Visible Grain
    • Some types of rocks don't even have any grain even when observed under a microscope
      • These types of rocks have no grains because when they form they cool very quickly giving no time to form grains
      • Because they have no grains, these types of rocks are smooth and shiny, just like the texture of a thick piece of glass
      • Other rocks form from extremely small pieces of silica that settle out of the water
        • For example: the rock flint forms in this type of manner

Mineral Composition

• Geologists often look very closely at a rock to find out its mineral composition
  • To identify a mineral, a geologist must do several things.
    • First the geologist has to cut the rock very thin so that light can shine through its crystals.
    • Then they do some tests to figure out the mineral's special properties.
      • One of the test that they do is that they test the rocks surface with acid so that they see whether a mineral is made up of compounds called carbonates.
      • Another test that they do is that they test the rock with a magnet to see if it has the minerals iron, nickel, or cobalt.

Origin

• There are three major types of groups of rocks.
  • They are igneous rocks, sedimentary rocks, metamorphic rocks.
    • Igneous rocks form from the cooling of lava above the surface or molten rock below the surface.
    • Sedimentary rocks form when sediments pile up under pressure.
    • Metamorphic rocks form when pressure is applied to sedimentary or igneous rocks.
      • Metamorphic rocks often form deep below the surface.